1. Field of the Invention:
The present invention relates to the production of whisker-free silicon nitride (Si.sub.3 N.sub.4) shaped articles by the carbonitriding of silica (SiO.sub.2).
2. Description of the Prior Art:
Silicon nitride is a member of that class of ceramics deemed "technical" materials. By reason of a unique combination of particular properties, it is a candidate of choice for producing articles which must retain their properties up to high temperatures (1,000.degree. to 1,200.degree. C.). These products can thus be used for the manufacture of carburetor plates, of rotors for turbo-compressors, and of precombustion chambers for diesel motors in the automotive industry, in particular because of high mechanical resistance, high temperature resistance, and its resistance to thermal shock and wear. It is equally useful for the manufacture of ball bearings, notably because of its low coefficient of friction and its resistance to wear, and also for the manufacture of pulverization tubes, extrusion plates, cutting tools, again due to its high durability and its resistance to wear.
Typically, shaped articles of silicon nitride are produced by high temperature sintering, under a nitrogen atmosphere, of a silicon nitride powder to which a densifying additive is added.
Currently, silicon nitride is itself prepared, on an industrial scale, according to one of the following mechanisms:
(1) Direct nitriding of silicon: EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4
The reaction is very exothermic and difficult to control and coarse powders are produced which must be ground into a fine powder, which is often present in the form of very angular grains;
(2) Decomposition of a silicon polyimide, for example according to the following mechanisms: EQU 3Si(NH).sub.2 .fwdarw.Si.sub.3 N.sub.4 +2NH.sub.3 EQU 3Si(NH.sub.2).sub.4 .fwdarw.Si.sub.3 N.sub.4 +8NH.sub.3
Such reactions produce a very fine but amorphous powder, which is then suitably crystallized at a temperature of from 1,300.degree. to 1,500.degree. C. in order to provide a powder well adopted for sintering.
Another process (3) for the synthesis of Si.sub.3 N.sub.4 entails the carboreduction or carbonitriding of silica (3SiO.sub.2 +6C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6CO), which is of interest because it requires only inexpensive starting materials. It is, however, inconvenient and rarely proceeds to completion (due to difficulties in attacking the last grains of silica). Also, such reaction must not be conducted at more than about 1,500.degree. C., to avoid forming SiC. This reaction produces a gaseous intermediate of silicon, of the species SiO, this species being capable of reacting, in gaseous phase (3SiO+3CO+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +3CO.sub.2), to produce Si.sub.3 N.sub.4 in a state which is no longer spherical, but rather in the form of whiskers. In the absence of this type of reaction, the SiO can be entrained and transported by nitrogen to the colder zones of the apparatus (about 1,000.degree. C.) and can be decomposed, thereby promoting fouling.
The control of the geometry of the grains, inter alia to avoid the formation of whiskers, and the formulation of a reaction mixture starting material are well described in the literature, patent and otherwise. For example:
FR-A-2,388,763 describes adding 5% to 50% by weight of a powder of Si.sub.3 N.sub.4 to a mixture of carbon and silica (beginning of crystallization, with a goal of rapidly producing a fine powder of Si.sub.3 N.sub.4);
EP-A-82,343 describes adding at least 50% of an initial Si.sub.3 N.sub.4 charge to itself provide for the carbonitriding and suffices to produce a fine and geometrically controlled powder;
JP-61/174,106 describes preparing a fine powder of Si.sub.3 N.sub.4 by means of a starting material selected from among SiC, Si.sub.3 N.sub.4, AlN, TiN, and TiC;
JP-63/176,301 and JP-63/239,104 describe the addition of an initial charge of .beta.-Si.sub.3 N.sub.4, thereby producing a powder rich in .beta.-Si.sub.3 N.sub.4 and poor in .alpha.-Si.sub.3 N.sub.4 ;
EP-A-131,894 describes incorporating a mixture of SiO.sub.2 /C, an initial crystallization charge (.alpha.-Si.sub.3 N.sub.4) and an additive selected from among the compounds of Mg, Be, Ca, Sr, Ge, Sn, Ti, Hf, Zr, thereby preparing powders which are fine and rich in .alpha.-Si.sub.3 N.sub.4 ;
NL-88/02,117 describes the formation of particulates having a density less than 0.8 g/cm.sup.3 and an increased reactivity;
DE-3,612,162 and JP-63/319,204 describe granulating the primary materials and fluidizing same by means of nitrogen (the '204 patent).
Indeed, even if the aforesaid various techniques improve or control the discharge or loss of silicon compounds or the size of the grains, it has not to date been possible, at the same time, to limit such losses of silicon compounds and control the particle size of the grains employing a simple reaction mixture.